Halliburton Co., the world’s second-largest oilfield-services provider, fell the most in five months after a government report said the company recommended to seal BP’s Gulf of Mexico well was unstable and may have contributed to the largest US offshore oil spill.

Halliburton, which reached a two-year high on Oct. 15, plunged 8 percent, or $2.74, to close at $31.68, with 97.8 million shares changing hands.

The stock fell after the National Commission on the BP Deepwater Horizon Oil Spill said documents provided by the Houston-based company showed at least three tests of the cement, in February and April, found the mixture for the doomed Macondo well unstable. BP received data from at least one test in March, the commission staff said in a letter yesterday.

“Halliburton and BP both had results in March showing that a very similar foam slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data,” according to a letter sent to the commissioners.